The discovery of metallocene catalysts activated with aluminoxanes has enabled the synthesis of new polyolefins with improved properties. A significant disadvantage of metallocene catalysts, however, is the requirement of large amounts of often expensive co-catalyst (such as an aluminoxane) to activate the catalysts. Additionally, while homogeneous metallocene catalysts can be used in solution phase reactors, the metallocene catalyst compounds generally need to be supported to be used in most other polymerization processes. Thus, while many metallocene catalysts are capable of making polyolefins with commercially desirable properties, the catalysts are often not practical or economical on an industrial scale due to the large amount of co-catalyst needed and difficulties in incorporating the catalyst and co-catalyst on a support.
It is important to find a way to incorporate the metallocene and co-catalyst onto the support without losing the advantages of the homogenous metallocene compound, including high catalyst activity, stereochemical control, and the ability to tailor polymer properties. Identifying the optimum properties for metallocene catalyst supports is an area of significant research interest. Both the nature of the support and the method used to integrate the support and/or co-catalyst can affect the catalyst activity and the final properties of the polymer.
Although aluminoxanes are expensive, silica supported catalysts with higher aluminoxane loadings are desirable in some circumstances. For example, when the metallocene compound has low activity or low activation efficiency or when a multi-catalyst precursor system is used where the total catalyst precursor loadings are higher than usual, higher aluminoxane loading may be required to achieve a commercially viable catalyst activity. In polymerization processes where liquid solvent is present, such as slurry and condensed mode processes, MAO is soluble in the solvent and can leach out of the silica particles. It is not possible with conventional silicas, e.g., Grace 948 or 955, PQ ES 70 or ES 757, to load more than about 8 to 9 mmol Al/g of silica onto the support without leaching of MAO (and possibly catalyst) into the solvent medium. This leaching can cause fouling and fines in the reactor system and can negatively impact catalyst activity and polymer properties.
It is also important for a catalyst support to be able to retain mechanical strength under the operating conditions of the process in which it is used. Many polymerization processes take place at significantly higher than ambient temperatures and pressures. If the mechanical strength of the support is compromised, the impregnated silica particles can fragment. This can also lead to activator and catalyst leaching into the solvent medium. Additionally, polymerization can start to take place on the smaller fragmented particles, leading to agglomerates within the reactor system that can cause fouling, plugging, and other problems.
Pullukat, T. J., et al., “Microspherical Silica Supports with High Pore Volume for Metallocene Catalysts,” presented at Metallocenes Europe '97, Dusseldorf, Germany, Apr. 8-9, 1997, pp. 1-11 discloses silica gel beads with a pore volume of 3.0 cc/g. It is said that the higher pore volumes of these silicas allow for greater versatility in preparing high surface area supports. Metallocene catalysts using these high pore volume silicas are disclosed with an MAO loading of 7 mmol Al/g silica.
U.S. Pat. No. 6,001,764 discloses a non-metallocene Ziegler-Natta based catalyst component on a silica support having high pore volume and high surface area. The catalytic component comprises a complex product of a transition metal halide and a metal alkyl which excludes cyclopentadienyl. The examples do not use an aluminoxane co-catalyst, and no mention is made of aluminoxane loading. See also, U.S. Pat. No. 6,855,783.
Thus, there is a need for catalyst systems, particularly metallocene catalyst systems, with supports having higher aluminoxane loading capabilities that are capable of maintaining the mechanical strength necessary for a variety of polymerization processes.